1. Field of the Invention
The present invention relates to a power supply circuit, and to a semiconductor integrated circuit device including the power supply circuit and a functional circuit powered by the power supply circuit.
2. Description of the Related Art
As the functionality of battery-powered electronic devices such as calculators, watches, and electronic toys keeps increasing, their power consumption also increases, making it necessary to take steps to extend their battery life. These steps include reducing the current consumption of their integrated circuit chips and in particular their microcomputer chips, through measures taken in the circuit design, layout design, and manufacturing stages.
In Japanese Patent Application Publication No. H7-38056, Uchiyama et al. disclose a semiconductor integrated circuit device with an embedded power supply circuit that receives an external supply voltage and generates an internal supply voltage suitable for the operating speed of the internal circuits of the device, and for the threshold voltage of the metal-oxide-semiconductor field-effect transistors in the internal circuits, so that the internal circuits can operate economically on the minimum necessary voltage despite variations in temperature, manufacturing process conditions, and other conditions.
Paragraph 0017 in the above publication describes a semiconductor integrated circuit device with a battery voltage detection circuit that compares a battery voltage with a reference voltage to be supplied to the logic circuits in the device, and a step-up/step-down voltage converter that steps the battery voltage up or down if it deviates too far from the reference voltage.
A problem with these conventional voltage conversion schemes is that the semiconductor integrated circuit devices used in many of today's products include a variety of circuits and their power requirements cannot always be determined from their operating speed. For example, there may be input-output (I/O) circuits with particular startup requirements and external interface requirements, oscillator circuits that do not require much voltage but are normally kept running constantly, and logic circuits that leak large amounts of current if operated at too low a voltage. It is unreasonable to control the power supply to all such circuits according to a single reference voltage or operating speed.